1. Field of the Invention
The present invention relates to a MOS type of solid-state imaging device and more particularly to a solid-state imaging device for use with a low-voltage-driven and moving-image-compatible CMOS sensor camera or the like.
2. Description of the Related Art
In recent years, a MOS type of solid-state imaging device using an amplification type of MOS sensor has been put into practice as one of the solid-state imaging devices. This solid-state imaging device, which is adapted to amplify a signal detected by a photodiode provided for each pixel through a MOS transistor, has a feature of high sensitivity.
The pixels of the MOS type of solid-state imaging device are each composed of a photodiode adapted to provide photoelectric conversion, a readout transistor adapted to read a signal, an amplifying transistor adapted to amplify the signal, a vertical select transistor adapted to select a read line, a reset transistor for resetting a signal charge, etc. The amplifying transistor has its source connected to a vertical signal line. A signal read onto a vertical signal line is output onto a horizontal signal line through a horizontal select transistor.
FIGS. 13 and 14 illustrate the structure of one pixel in a conventional MOS type of solid-state imaging device disclosed in, for example, Japanese Unexamined Patent Publication NO. 2000-91552. FIG. 13 is a plan view and FIG. 14 is a sectional view. In these figures, 80 denotes a p-type substrate, 81 a photoelectric conversion section (n-type layer) which serves as an embedded photodiode, 82 a device isolation layer, 83 a scanning transistor well (p-type layer), 84 a signal storage section (n-type layer), 85 a surface shield layer (p-type layer), 93 the gate of a signal readout transistor, 94 the gate of an amplifying transistor, 95 the gate of an address transistor, 96 the gate of a reset transistor, 97 the source/drain of a scanning transistor, and 98 an interconnect line.
However, this type of device has such problems as will be explained below with reference to FIGS. 15A, 15B, and 15C which show internal potential profiles in the sectional structure of FIG. 14. That is, in the sectional structure shown in FIG. 15A, as shown in FIG. 15B, the part which is deepest in potential in the photoelectric conversion section 81 including the signal storage section 84 is located outside the readout gate 93. For this reason, when the readout gate 93 is driven with a low voltage, a potential pocket is produced in the signal storage section 81 and some charge C remains in this pocket as shown in FIG. 15C. This means that all the signal charge produced as the result of receiving light cannot be read out with certainty. That is, a potential barrier produced at the side of the readout gate does not allow a part C of signal charge produced in the photoelectric conversion section 81 to be transferred, resulting in a residual image.
One might suggest extending a part of the signal storage section 84 to under the readout gate 93. However, since the potential profile is affected not only by the signal storage section 84 but also by the photoelectric conversion section 81, the part of the deepest potential would be positioned under the photoelectric conversion section 81 even if a part of the signal storage section 81 in the structure of FIG. 14 were extended to under the readout gate 93. Thus, the deepest potential part would still be positioned outside the readout gate 93. Therefore, this approach cannot avoid the problem of potential barrier.
A method has been proposed which forms the readout gate to conform to the position of deepest potential in the corresponding photodiode (see, for example, Japanese Unexamined Patent Publication No. 11-274463). With this method, however, the readout gate electrode overhangs the light receiving section of the corresponding pixel, blocking a part of the light path. With this method, therefore, the residual image is lowered but there arises a problem of shading in which a change in color occurs in the periphery of an image due to misalignment occurring in processing steps.
Thus, in the conventional low-voltage-driven MOS type of solid-state imaging device, a packet of signal charge produced by a photoelectric conversion section cannot be transferred in its entirety to the signal scanning circuit, thereby causing a residual image and kTc noise.